The process of the present invention for the synthesis of cerium oxide nano-particles, may be classified into two (2) categories according to whether water is involved in the synthetic process or not. In the first category of the process of the present invention, cerium oxide nanocrystals are synthesized through a non-hydrolytic sol-gel reaction of a mixture which comprises an anhydrous cerium precursor, a surfactant and ether. In the second category of the process according to the present invention, cerium oxide nanocrystals are synthesized via a hydrolytic sol-gel reaction of a mixture which comprises a hydrous cerium precursor and a surfactant. The reaction mixture of the sol-gel reaction of the present invention may further comprise organic solvent.
The particle sizes and their size distribution, and shapes of cerium oxide nanocrystals synthesized according to the process of the present invention, may be controlled by varying cerium precursors, reaction times, reaction temperatures or surfactants.
For the last 20 years, nanocrystals have been intensively studied, not only for their fundamental scientific interest derived from their size-dependent properties, but also for their many technological applications. These nanocrystalline materials exhibit novel electronic, magnetic, optical, chemical and mechanical properties that cannot be achieved in their bulk counterparts.
Cerium oxide (CeO2) is a rare earth oxide that has attracted a great deal of interest due to its unique properties, including its high mechanical strength, oxygen ion conductivity, and oxygen storage capacity. Because of these characteristics, cerium oxide has been extensively used as oxygen ion conductors in solid oxide fuel cells, oxygen pumps and amperometric oxygen monitors. Cerium oxide nanocrystals have been used as a polishing agent for the chemical mechanical planarization (CMP) process in semiconductor fabrication processes.
Up to now, several processes have been studied and developed to synthesize cerium oxide nanocrystals. These processes include coprecipitation method [Atul S. Deshpande, Nicola Pinna, Pablo Beato, Markus Antonietti, and Markurs Niederberger “Synthesis and Characterization of Stable and Crystalline Ce1-xZrxO2 Nanoparticle Sols” Chem. Mater. 2004, 16, 2599], solvothermal process [Masashi Inoue, Minoru Kimura, and Tomoyuki Inui “Transparent colloidal solution of 2 nm ceria particles” Chem. Commun. 1999, 957], reverse micelle method [Toshiyuki Masui, Kazuyasu Fujiwara, Ken-ichi Machida, and Gin-ya Adachi “Characterization of Cerium(IV) Oxide Ultrafine Particles Prepared Using Reversed Micelles” Chem. Mater. 1997, 9, 2197], and sonochemical method [Lunxiang Yin, Yanqin Wang, Guangsheng Pang, Yuri Koltypin, and Aharon Gedanken “Sonochemical Synthesis of Cerium Oxide Nanoparticles—Effect of Additives and Quantum Size Effect” J. Colloid Interface Sci. 2002, 246, 78].
Vioux and coworkers summarized many non-hydrolytic sol-gel reactions for the synthesis of oxide materials [Vioux, A “Nonhydrolytic Sol-Gel Routes to Oxides” Chem. Mater, 1997, Vol. 9, 2292].
Also, Joo et al. reported the synthesis of uniform-sized tetragonal zirconium oxide nanocrystals on the multi-gram scale via a non-hydrolytic sol-gel processes [Jin Joo, Taekyung Yu, Young Woon Kim, Hyun Min Park, Fanxin Wu, Jin Z. Zhang, and Taeghwan Hyeon, “Multi-gram Scale Synthesis and Characterization of Monodisperse Tetragonal Zirconia Nanocrystals,” J. Am. Chem. Soc. 2003, 125, 6553]. In this paper, zirconia nanocrystals were produced using alkyl halide elimination non-hydrolytic sol-gel reaction in the presence of surfactant.
Very recently, cerium oxide nanocrystals having diameters of 30 nm˜250 nm were synthesized via a thermal decomposition of cerium salts at a temperature of 400° C. to 1200° C. [WO2004/037722].
Unfortunately, the cerium oxide nanocrystals synthesized through the previous processes, have the following weak points for commercial applications.
Firstly, the cerium oxide nanocrystals larger than 80 nm synthesized through the conventional process, may increase the probability of the outbreak of scratch during the chemical mechanical planarization (CMP) processes.
Secondly, a time-consuming and laborious size selection process is required to obtain uniform cerium oxide nanocrystals since cerium oxide nanocrystals synthesized through the conventional process, are highly polydisperse.
Thirdly, the amount of the cerium oxide nanocrystals which can be obtained through the conventional process, is only a few milli-grams, which is not suitable amount for large-scale industrial applications.
Therefore, an object of the present invention is to provide a large-scale synthetic process to obtain cerium oxide nanocrystals having a narrow particle size distribution through an inexpensive process and by using non-toxic reagents in order to overcome the afore-mentioned deficiencies of the prior arts. That is, it is the primary object to provide a process for the preparation of cerium oxide nanocrystals via a non-hydrolytic sol-gel reaction, which comprises the steps for: i) the formation of a cerium-surfactant complex by heating the mixture of a cerium precursor and a surfactant under reduced pressure; and ii) aging of said cerium-surfactant complex in an etheric compound at a predetermined temperature.
A still another object of the present invention is to provide a process for the preparation of cerium oxide nanocrystals through a hydrolytic sol-gel reaction, which comprises the steps for: i) the formation of a cerium-surfactant complex by heating the mixture of a cerium precursor and a surfactant under atmospheric pressure; and ii) aging of said cerium-surfactant complex in an etheric compound at a predetermined temperature.